a. Field of the Invention
This invention relates to a loud-speaker, and, more particularly, it is concerned with a high output loud-speaker for reproducing medium and low-pitched sounds.
b. Description of Prior Arts
Heretofore, this kind of loud-speaker is so constructed that a plurality of loud-speaker units 5, with the area of a vibrating plate or a diaphragm of each speaker unit being made wide or highly compliant, are arranged in a row on the front baffle board of a speaker box B of a closed type or a bass-reflex type, or a plurality of loud speaker units are disposed on a flat surface of a broad baffle plate, as shown in FIG. 1 of the accompanying drawing.
A level of an output sound pressure in this type of loud-speaker is usually defined in terms of "output sound pressure level of each speaker unit .times. number of the speaker units used". In reality, however, the electrical signal applied to the entire speaker units is not effectively converted to sound on account of the sound intereference which takes place in the diaphragm of each speaker unit, and mutual sound interferences among the speaker units which take place in front of the baffle board.
Also, as shown in FIG. 2 of the accompanying drawing, a loud-speaker of a construction, in which a sound horn H is fitted in front of a speaker unit having a large mouth diameter causes partitioning vibration between the surrounding part of the diaphragm and the center part thereof to a higher frequency side even in its low-pitched sound region, because the area of the diaphragm of the speaker unit 5 is large. On account of this, it occurs from time to time that the diaphragm does not bring about perfect piston vibration with respect to the sounding horn H. In other words, the throat diameter of the sounding horn is too large in comparison with the diaphragm part to obtain the effective horning effect.
Generally speaking, the sound pressure at the low-pitched sound region of a loud-speaker unit tends to attenuate in a frequency range lower than 100 to 200 Hz. On account of this, when reproduction of the low-pitched sound is particularly regarded as important, it has so far been the usual practice to use a loud-speaker unit of a large mouth diameter or a plurality of small-diameter speaker units in combination with such large-diameter speaker unit for obtaining required low-pitched sound pressure.
However, the loud-speaker unit of the large diameter has a large mass in its vibration system, on account of which it has such a property that its reproduction frequency band becomes narrow, hence its transition characteristic is also poor. Moreover, even if a plurality of small-diameter speaker units are combined with the large-diameter speaker unit, it is still impossible to perfectly solve the problem of attenuation of the sound pressure in the low-pitched sound region. In addition, since a plurality of the loud-speaker units are fitted on the baffle board in a predetermined arrangement, there inevitably takes place in front of the baffle board the interferences among the sounds from each of the loud-speaker units with the consequence that reproduction of the low-pitched sound is insufficient.
For improvement in the abovementioned points, there has been known a phase-inverting type loud-speaker (generally called "bass-reflex" type loud-speaker unit). Even in this type of loud-speaker, the low-pitched sound from the speaker units and the low-pitched sound from the phase-inverting duct mutually interfere to make it still unable to reach a condition where sufficient reproduction of the low-pitched sound is realized, because a space interval between the phase-inverting duct and the speaker units is made three times or more as long as the effective vibrating radius of the speaker unit.